I don't know. But I don't think that it would change much in the big picture; while a few would see Mark "I've failed the Turing test" Zuckenberg as a saviour, some are so disgusted at him and Facebook that they'd leave ASAP, and the demon that you know is better than the one that you don't.
Nothing really. The site is dying and Zuck can’t pump new life into it. If he gave everyone on Facebook a free Reddit account, it would probably be barely used.
I think once he applied a bunch of rules and bullshit data harvesting it would basically be the same thing as Facebook and people wouldn't see the point of having a separate Reddit account. Probably no one would use it
Also after all the shitty things Steve Huffman has done to Reddit it basically has no value anymore. He's devalued the brand so much that even the name Reddit is now synonymous with corporate greed
Also most people on Facebook who don't already have a Reddit account think Reddit is just the scary and complicated version of Facebook anyway, so there's really no reason why they would want to use it